EDEN PRAIRIE, Minn. -- It appears Chris Cook finally is going to get his chance.

With starting cornerback Asher Allen recovering from a concussion and listed as doubtful, Cook is expected to make his first start with the Minnesota Vikings' base defense on Sunday at Chicago.

"This is my chance, and I'm definitely not looking back," Cook said. "I'm attacking it with full force right now."

It's an opportunity that seemed inevitable in late August, when the Vikings' top draft pick was performing well enough to surpass Allen and veteran Lito Sheppard for the starting nod in the all-important third preseason game.

But it was in that Aug. 28 game against Seattle that Cook suffered the first of two knee injuries that required minor surgery, sidelined him for four of the Vikings' first five regular-season games and reduced him to subpackage duty once he returned.

Even in that limited role, Cook has scuffled at times, getting torched in his first game back Oct. 24 at Green Bay and splitting nickel duties again the following week against New England. On Thursday, Childress and defensive coordinator Leslie both acknowledged the rookie's knees still seem to be giving him some trouble.

Cook did hold up OK while finishing Sunday's win over Arizona in Allen's place, however, and he said it's just been a matter of improving his conditioning and getting enough snaps to regain his old form. He's been doing extra running after practice to get back into shape and pronounced himself ready to take a full game's worth of work.

"It takes a lot of repetitions to get up to the speed," Cook said. "It's a fast game, lot of fast guys everywhere you look. When I first got back, it was just my football. Just took a little time."

Unless Allen suits up, Sheppard likely will get the first shot in the nickel defense, though Childress indicated Frank Walker would rotate in as well.

Allen's health

Allen said he suffered what he termed a "mild" concussion while tackling Cardinals running back Tim Hightower late in the third quarter on Sunday -- not on the next play, when he appeared unsteady after taking down receiver Larry Fitzgerald.

Though he said he's passed the post-concussion tests, he continues to take an exercise test daily and understands team doctors won't let him return until he's 100% recovered.

"You've just got to take it day by day," Allen said. "That's really what we're doing."

Preparing for Peppers

The last time Bryant McKinnie faced All-Pro pass rusher Julius Peppers, he ended up on the bench in the second half -- but the Vikings' left tackle said he hasn't reviewed the tape of that Dec. 20 debacle in Carolina.

"I had hurt my ankle the week before and had plantar fasciitis last time," McKinnie said, "so I feel better this time around than I did back then. I just did what I had to do. This time around, I'm definitely in better shape."

Instead, McKinnie said he's focused on how Peppers is attacking opponents this season. The Bears shift him from side to side based on where the tight is aligned -- Peppers generally plays the "open" side -- but McKinnie said he expects the bulk of the matchup.

"I don't need to go through last year's game," McKinnie said. "I just need to see what he's done this year and study those moves."